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NAME
List::Util - A selection of general-utility list subroutines
SYNOPSIS
use List::Util qw(first max maxstr min minstr reduce shuffle sum);
DESCRIPTION
"List::Util" contains a selection of subroutines that people have expressed would be nice
to have in the perl core, but the usage would not really be high enough to warrant the use
of a keyword, and the size so small such that being individual extensions would be
wasteful.
By default "List::Util" does not export any subroutines.
LIST-REDUCTION FUNCTIONS
The following set of functions all reduce a list down to a single value.
$result = reduce { BLOCK } @list
Reduces @list by calling "BLOCK" in a scalar context multiple times, setting $a and $b
each time. The first call will be with $a and $b set to the first two elements of the
list, subsequent calls will be done by setting $a to the result of the previous call and
$b to the next element in the list.
Returns the result of the last call to the "BLOCK". If @list is empty then "undef" is
returned. If @list only contains one element then that element is returned and "BLOCK" is
not executed.
The following examples all demonstrate how "reduce" could be used to implement the other
list-reduction functions in this module. (They are not in fact implemented like this, but
instead in a more efficient manner in individual C functions).
$foo = reduce { defined($a) ? $a :
$code->(local $_ = $b) ? $b :
undef } undef, @list # first
$foo = reduce { $a > $b ? $a : $b } 1..10 # max
$foo = reduce { $a gt $b ? $a : $b } 'A'..'Z' # maxstr
$foo = reduce { $a < $b ? $a : $b } 1..10 # min
$foo = reduce { $a lt $b ? $a : $b } 'aa'..'zz' # minstr
$foo = reduce { $a + $b } 1 .. 10 # sum
$foo = reduce { $a . $b } @bar # concat
$foo = reduce { $a || $code->(local $_ = $b) } 0, @bar # any
$foo = reduce { $a && $code->(local $_ = $b) } 1, @bar # all
$foo = reduce { $a && !$code->(local $_ = $b) } 1, @bar # none
$foo = reduce { $a || !$code->(local $_ = $b) } 0, @bar # notall
# Note that these implementations do not fully short-circuit
If your algorithm requires that "reduce" produce an identity value, then make sure that
you always pass that identity value as the first argument to prevent "undef" being
returned
$foo = reduce { $a + $b } 0, @values; # sum with 0 identity value
The remaining list-reduction functions are all specialisations of this generic idea.
any
my $bool = any { BLOCK } @list;
Since version 1.33.
Similar to "grep" in that it evaluates "BLOCK" setting $_ to each element of @list in
turn. "any" returns true if any element makes the "BLOCK" return a true value. If "BLOCK"
never returns true or @list was empty then it returns false.
Many cases of using "grep" in a conditional can be written using "any" instead, as it can
short-circuit after the first true result.
if( any { length > 10 } @strings ) {
# at least one string has more than 10 characters
}
all
my $bool = all { BLOCK } @list;
Since version 1.33.
Similar to "any", except that it requires all elements of the @list to make the "BLOCK"
return true. If any element returns false, then it returns false. If the "BLOCK" never
returns false or the @list was empty then it returns true.
none
notall
my $bool = none { BLOCK } @list;
my $bool = notall { BLOCK } @list;
Since version 1.33.
Similar to "any" and "all", but with the return sense inverted. "none" returns true only
if no value in the @list causes the "BLOCK" to return true, and "notall" returns true only
if not all of the values do.
first
my $val = first { BLOCK } @list;
Similar to "grep" in that it evaluates "BLOCK" setting $_ to each element of @list in
turn. "first" returns the first element where the result from "BLOCK" is a true value. If
"BLOCK" never returns true or @list was empty then "undef" is returned.
$foo = first { defined($_) } @list # first defined value in @list
$foo = first { $_ > $value } @list # first value in @list which
# is greater than $value
max
my $num = max @list;
Returns the entry in the list with the highest numerical value. If the list is empty then
"undef" is returned.
$foo = max 1..10 # 10
$foo = max 3,9,12 # 12
$foo = max @bar, @baz # whatever
maxstr
my $str = maxstr @list;
Similar to "max", but treats all the entries in the list as strings and returns the
highest string as defined by the "gt" operator. If the list is empty then "undef" is
returned.
$foo = maxstr 'A'..'Z' # 'Z'
$foo = maxstr "hello","world" # "world"
$foo = maxstr @bar, @baz # whatever
min
my $num = min @list;
Similar to "max" but returns the entry in the list with the lowest numerical value. If the
list is empty then "undef" is returned.
$foo = min 1..10 # 1
$foo = min 3,9,12 # 3
$foo = min @bar, @baz # whatever
minstr
my $str = minstr @list;
Similar to "min", but treats all the entries in the list as strings and returns the lowest
string as defined by the "lt" operator. If the list is empty then "undef" is returned.
$foo = minstr 'A'..'Z' # 'A'
$foo = minstr "hello","world" # "hello"
$foo = minstr @bar, @baz # whatever
product
my $num = product @list;
Since version 1.35.
Returns the numerical product of all the elements in @list. If @list is empty then 1 is
returned.
$foo = product 1..10 # 3628800
$foo = product 3,9,12 # 324
sum
my $num_or_undef = sum @list;
Returns the numerical sum of all the elements in @list. For backwards compatibility, if
@list is empty then "undef" is returned.
$foo = sum 1..10 # 55
$foo = sum 3,9,12 # 24
$foo = sum @bar, @baz # whatever
sum0
my $num = sum0 @list;
Since version 1.26.
Similar to "sum", except this returns 0 when given an empty list, rather than "undef".
KEY/VALUE PAIR LIST FUNCTIONS
The following set of functions, all inspired by List::Pairwise, consume an even-sized list
of pairs. The pairs may be key/value associations from a hash, or just a list of values.
The functions will all preserve the original ordering of the pairs, and will not be
confused by multiple pairs having the same "key" value - nor even do they require that the
first of each pair be a plain string.
pairgrep
my @kvlist = pairgrep { BLOCK } @kvlist;
my $count = pairgrep { BLOCK } @kvlist;
Since version 1.29.
Similar to perl's "grep" keyword, but interprets the given list as an even-sized list of
pairs. It invokes the "BLOCK" multiple times, in scalar context, with $a and $b set to
successive pairs of values from the @kvlist.
Returns an even-sized list of those pairs for which the "BLOCK" returned true in list
context, or the count of the number of pairs in scalar context. (Note, therefore, in
scalar context that it returns a number half the size of the count of items it would have
returned in list context).
@subset = pairgrep { $a =~ m/^[[:upper:]]+$/ } @kvlist
As with "grep" aliasing $_ to list elements, "pairgrep" aliases $a and $b to elements of
the given list. Any modifications of it by the code block will be visible to the caller.
pairfirst
my ( $key, $val ) = pairfirst { BLOCK } @kvlist;
my $found = pairfirst { BLOCK } @kvlist;
Since version 1.30.
Similar to the "first" function, but interprets the given list as an even-sized list of
pairs. It invokes the "BLOCK" multiple times, in scalar context, with $a and $b set to
successive pairs of values from the @kvlist.
Returns the first pair of values from the list for which the "BLOCK" returned true in list
context, or an empty list of no such pair was found. In scalar context it returns a simple
boolean value, rather than either the key or the value found.
( $key, $value ) = pairfirst { $a =~ m/^[[:upper:]]+$/ } @kvlist
As with "grep" aliasing $_ to list elements, "pairfirst" aliases $a and $b to elements of
the given list. Any modifications of it by the code block will be visible to the caller.
pairmap
my @list = pairmap { BLOCK } @kvlist;
my $count = pairmap { BLOCK } @kvlist;
Since version 1.29.
Similar to perl's "map" keyword, but interprets the given list as an even-sized list of
pairs. It invokes the "BLOCK" multiple times, in list context, with $a and $b set to
successive pairs of values from the @kvlist.
Returns the concatenation of all the values returned by the "BLOCK" in list context, or
the count of the number of items that would have been returned in scalar context.
@result = pairmap { "The key $a has value $b" } @kvlist
As with "map" aliasing $_ to list elements, "pairmap" aliases $a and $b to elements of the
given list. Any modifications of it by the code block will be visible to the caller.
See "KNOWN BUGS" for a known-bug with "pairmap", and a workaround.
pairs
my @pairs = pairs @kvlist;
Since version 1.29.
A convenient shortcut to operating on even-sized lists of pairs, this function returns a
list of ARRAY references, each containing two items from the given list. It is a more
efficient version of
@pairs = pairmap { [ $a, $b ] } @kvlist
It is most convenient to use in a "foreach" loop, for example:
foreach my $pair ( pairs @KVLIST ) {
my ( $key, $value ) = @$pair;
...
}
Since version 1.39 these ARRAY references are blessed objects, recognising the two methods
"key" and "value". The following code is equivalent:
foreach my $pair ( pairs @KVLIST ) {
my $key = $pair->key;
my $value = $pair->value;
...
}
pairkeys
my @keys = pairkeys @kvlist;
Since version 1.29.
A convenient shortcut to operating on even-sized lists of pairs, this function returns a
list of the the first values of each of the pairs in the given list. It is a more
efficient version of
@keys = pairmap { $a } @kvlist
pairvalues
my @values = pairvalues @kvlist;
Since version 1.29.
A convenient shortcut to operating on even-sized lists of pairs, this function returns a
list of the the second values of each of the pairs in the given list. It is a more
efficient version of
@values = pairmap { $b } @kvlist
OTHER FUNCTIONS
shuffle
my @values = shuffle @values;
Returns the values of the input in a random order
@cards = shuffle 0..51 # 0..51 in a random order
KNOWN BUGS
RT #95409
<https://rt.cpan.org/Ticket/Display.html?id=95409>
If the block of code given to "pairmap" contains lexical variables that are captured by a
returned closure, and the closure is executed after the block has been re-used for the
next iteration, these lexicals will not see the correct values. For example:
my @subs = pairmap {
my $var = "$a is $b";
sub { print "$var\n" };
} one => 1, two => 2, three => 3;
$_->() for @subs;
Will incorrectly print
three is 3
three is 3
three is 3
This is due to the performance optimisation of using "MULTICALL" for the code block, which
means that fresh SVs do not get allocated for each call to the block. Instead, the same SV
is re-assigned for each iteration, and all the closures will share the value seen on the
final iteration.
To work around this bug, surround the code with a second set of braces. This creates an
inner block that defeats the "MULTICALL" logic, and does get fresh SVs allocated each
time:
my @subs = pairmap {
{
my $var = "$a is $b";
sub { print "$var\n"; }
}
} one => 1, two => 2, three => 3;
This bug only affects closures that are generated by the block but used afterwards.
Lexical variables that are only used during the lifetime of the block's execution will
take their individual values for each invocation, as normal.
SUGGESTED ADDITIONS
The following are additions that have been requested, but I have been reluctant to add due
to them being very simple to implement in perl
# How many elements are true
sub true { scalar grep { $_ } @_ }
# How many elements are false
sub false { scalar grep { !$_ } @_ }
SEE ALSO
Scalar::Util, List::MoreUtils
COPYRIGHT
Copyright (c) 1997-2007 Graham Barr <gbarr@pobox.com>. All rights reserved. This program
is free software; you can redistribute it and/or modify it under the same terms as Perl
itself.
Recent additions and current maintenance by Paul Evans, <leonerd@leonerd.org.uk>.